1. Field of the Invention
The present invention relates to a process for forming a device isolation region of a trench structure.
2. Related Art
With reference to FIGS. 5(a) to 8, a conventional device isolation technique utilizing shallow trenches will be explained hereinbelow.
First, as shown in FIG. 5(a), a pad oxide film 12 of 50 to 200 .ANG. thick and a silicon nitride film 13 of 1000 to 2000 .ANG. thick are formed on a silicon substrate 11. Then, a resist pattern is formed to cover an active region by a lithography process. Using the resist pattern as a mask, the silicon nitride film 13 and the pad oxide film 12 in a region for forming a device isolation region are removed by dry etching. Then, a trench is formed in the silicon substrate 11 by dry etching as shown in FIG. 5(b).
Then, as shown in FIG. 5(c), the bottom and the sides of the trench formed in the silicon substrate 11 are oxidized by a thickness of 50 to 500 .ANG. at a temperature of 900 to 1100.degree. C. By the oxidization, an opening of the trench in the silicon substrate 11 is rounded and the sides and the bottom of the silicon substrate 11 are covered with a protective film 14 of a silicon oxide film. Then, as shown in FIG. 6(a), an oxide film 15 is deposited by CVD to completely fill the trench. Then, as shown in FIG. 6(b), the buried oxide film 15 is flattened by chemical mechanical polishing. Next, the silicon nitride film 13 and the pad oxide film 12 are removed as shown in FIG. 6(c). A gate oxide film 16 is formed, and a gate electrode material 17 is deposited thereon as seen in FIG. 7(a) and patterned into a gate electrode 17a by lithography and dry etching.
According to the above-described process, an edge of the opening of the trench formed in the silicon substrate 11 (indicated by "a" in FIG. 8) takes a sharp and almost right-angled form. The edge of the trench opening is optimized by oxidizing the edge in a certain oxidizing atmosphere at a certain oxidizing temperature after the formation of the trench so that the edge "a" is rounded. Further, the oxidization forms the protective film 14 of a silicon oxide film on the surface of the trench. After the later chemical mechanical polishing, the thickness of the flattened buried oxide film 15 filled in the trench is reduced by removal of the silicon nitride film 13 and the pad oxide film 12. As a result, a top portion of the buried oxide film in the device isolation region becomes lower than the surface of the silicon substrate in the active region as shown in FIG. 6(c).
When the gate oxide film and the gate electrode are formed thereafter, a channel region at a final stage extends to include the edge of the trench opening as shown in FIG. 7(b). Accordingly, if the edge is not sufficiently rounded, a gate electric field strengthens at this portion when a MOS transistor is operated and the transistor assumes apparent characteristics as if parasitic MOS transistors having much lower threshold value are arranged in parallel. Further, in a non-volatile memory utilizing the gate oxide film 16 as a tunnel oxide film, an F-N tunneling electric field increases in the edge region. Therefore an F-N tunneling current increases locally, and reliability in rewriting the non-volatile memory is deteriorated. Further, a thickness of the gate oxide film is reduced in the region where the edge is not sufficiently rounded, which further accelerates the above problem.
Further, in the above process, the optimization of the edge of the trench opening needs to be performed in one oxidization after the trench is formed. Therefore, to oxidize the edge for better roundness, the edge must be oxidized to a greater extent in one time. However, in case where the oxidization is performed after a certain amount of the oxide film has been formed, the silicon oxide film formed by oxidizing silicon increases in volume, which causes internal stress to accumulate in the edge region of the trench opening. Therefore, it is problematic because the device characteristics are adversely affected, the profile of the edge is varied due to a remarkable increase of the stress immediately after the oxidization or variations in the profile are increased.